The normal tubular heating element comprises a rectilinear metal sheath, usually round in cross section, and having a helically-coiled resistance wire within the sheath and held centered within the sheath by powdered refractory material. Terminal pins are connected to opposite ends of the resistor coil and extend outwardly of opposite ends of the sheath. Dielectric plugs surround the terminal pins at the sheath ends to contain the refractory material. U.S. Pat. No. 2,272,282, issued to E. L. Wiegand, illustrates a tubular heater of the general type referred to.
In normal manufacture, the tubular element above described is subjected to a rolling operation to reduce the diameter thereof and thus compress and densify the refractory material to a rock-like hardness. The rolling operation increases the length of the tubular element as the diameter of the tubular unit is reduced and the present invention is for the purpose of automatically measuring the growth of a sample length of the tubular element and for automatically making machine adjustments to hold the growth of the entire element within acceptable limits. In another aspect of the invention the full length of the tubular element is subjected to a measuring operation so that the full growth is measured.
In the manufacture of well-known surface units, a tubular element, after rolling, is put into a spiralling machine to provide a finished unit having a flat intermediate spiralled portion and a pair of terminal portions extending from opposite ends of the spiralled portion. A typical surface unit is shown in U.S. Pat. No. 3,350,674, issued to L. S. Kozbelt and D. M. Cunningham.
It is important that the final length of a rolled tubular element be held within certain tolerances, since if the rolled element is too long or too short, the terminal portions of the spiralled surface unit will be too long or too short or uneven in length and this causes problems, especially in a surface unit of the plug-in type. If the rolled element is too short, the spiralled unit cannot be used for plug-in purposes and most likely would be scrapped. If the rolled element is too long, the longer terminal portions may be trimmed to proper length, but this represents additional labor. If the terminal portions are uneven in length, with the shorter one within acceptable limits, the longer one may be trimmed but again this requires additional labor. The present invention provides for measuring a sample length of a tubular unit (or the full length) after having been reduced in the reducing rolls during the rolling operation, and utilizing the measurement in length growth to effect means for holding the tubular unit within prescribed tolerances.